BDNF Induces Calcium Elevations Associated With IBDNF, a Nonselective Cationic Current Mediated by TRPC Channels

Brain-derived neurotrophic factor (BDNF) has potent actions on hippocampal neurons, but the mechanisms that initiate its effects are poorly understood. We report here that localized BDNF application to apical dendrites of CA1 pyramidal neurons evoked transient elevations in intracellular Ca2+ concentration, which are independent of membrane depolarization and activation of N-methyl-d-aspartate receptors (NMDAR). These Ca2+ signals were always associated with IBDNF, a slow and sustained nonselective cationic current mediated by transient receptor potential canonical (TRPC3) channels. BDNF-induced Ca2+ elevations required functional Trk and inositol-tris-phosphate (IP3) receptors, full intracellular Ca2+ stores as well as extracellular Ca2+, suggesting the involvement of TRPC channels. Indeed, the TRPC channel inhibitor SKF-96365 prevented BDNF-induced Ca2+ elevations and the associated IBDNF. Thus TRPC channels emerge as novel mediators of BDNF-induced intracellular Ca2+ elevations associated with sustained cationic membrane currents in hippocampal pyramidal neurons.

Stretch induced accumulation of total Ca and Na in cytosol and nucleus: a comparison between cardiac trabeculae and isolated myocytesThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell.

By means of electron probe microanalysis (EPMA), we quantified changes in total sodium [Na] and calcium [Ca] concentration owing to the following: (i) local axial stretch (LAS) of isolated rat myocytes and (ii) end-to-end stretch (ETES) of rat ventricular trabeculae. For LAS, the distance between patch pipette and a cell-attached stylus was increased by maximally 20%; this activated a nonselective cationic current ISAC of approximately –0.5 nA, which was blocked by streptomycin. Trabeculae were stretched end-to-end from 85% Lmax to Lmax. Stretch increased cytosolic [Na]total by 34% in isolated myocytes (p < 0.001) and by 43% in trabeculae (p < 0.001). The increment in nuclear [Na]total was 21% in myocytes (p < 0.01) and 20% in trabeculae (p < 0.001). Stretch increased [Ca]total in isolated myocytes, in both cytosol (from 0.63 ± 0.09 to 1.09 ± 0.20 mmol/L, p < 0.05) and nucleus (from 0.33 ± 0.05 to 0.64 ± 0.13 mmol/L, p < 0.05). In trabeculae, the stretch-induced increment of 51% in cytosolic [Ca]total remained nonsignificant (p < 0.15). In the nucleus, [Ca]total did not change. We interpret the difference of stretch on nuclear calcium in myocytes vs. trabeculae with the assumption that LAS, but not ETES, produces shear-stress components that translate the mechanical stimulus deeply into the cell where it may modulate [Ca]total by signals independent of ISAC.

Nonselective cationic currents activated by acetylcholine in swine tracheal smooth muscle cells

Membrane currents in isolated swine tracheal smooth muscle cells were investigated using a pipette solution containing BAPTA-Ca2+ buffer and Cs+ as the major cation. With a pipette solution containing 100 nM free Ca2+, acetylcholine (ACh; 1-100 µM), in a concentration-dependent manner, activated a current without inducing shortening of cells, although neither 1 mM histamine nor 1 µM leukotriene D4 activated the current (n = 7, n is the number of cells). The effect of 100 µM ACh was suppressed by pretreatment with 100 µM atropine (n = 6) or intracellular application of preactivated pertussis toxin at a concentration of 0.1 µg·mL-1 (n = 8). Genistein (0.1-100 µM), in a concentration-dependent manner, suppressed the activation of the inward current by 100 µM ACh, whereas it did not significantly suppress that of the outward current (n = 6-8). With a pipette solution containing 50 nM free Ca2+, outward current, but not inward current, was activated by 100 µM ACh (n = 10). When the pipette solution had free Ca2+ concentrations greater than 50 nM, the inward current together with the outward current was activated. The ratio between the amplitude of the inward and outward currents was significantly increased as the free Ca2+ concentration in the pipette solution increased. The steady-state activation curve of the ACh-activated current with the 50 nM free Ca2+ pipette solution was fitted by a single Boltzmann distribution (Vh = +69.8 mV, k = -11.9 mV, n = 10). The activation time constant became smaller as the membrane potential was more depolarized (164.3 ± 5.9 ms at +40 mV to 92.4 ± 6.3 ms at +120 mV, n = 10). The reversal potential was not significantly changed by reducing extracellular Cl- concentration to one-tenth of the control (n = 8), suggesting that the current is a nonselective cationic current. These results suggest that ACh activates an outward nonselective cationic current via pertussis toxin-sensitive G-protein(s) coupled with muscarinic receptors. Involvement of genistein-sensitive tyrosine kinase in the activation process of the current is unlikely.Key words: tracheal smooth muscle, nonselective cationic current, acetylcholine, Ca2+ dependency, genistein sensitivity.

Ca2+-Activated Nonselective Cationic Current (I CAN) in Turtle Motoneurons

The presence of a calcium-activated nonspecific cationic (CAN) current in turtle motoneurons and its involvement in plateau potentials, bistability, and windup was investigated by intracellular recordings in a spinal cord slice preparation. In the presence of tetraethylammonium (TEA) and tetrodotoxin (TTX), calcium action potentials evoked by depolarizing current pulses were always followed by an afterdepolarization associated with a decrease in input resistance. The presence of the afterdepolarization depended on the calcium spike and not on membrane potential. Replacement of extracellular sodium by choline or N-methyl-d-glucamine (NMDG) reduced the afterdepolarization, confirming that it was mediated by a CAN current. Plateau potentials and windup were evoked in response to intracellular current pulses in the presence of agonist for different metabotropic receptors. Replacement of extracellular sodium by choline or NMDG did not abolish the generation of plateau potentials, bistability, or windup, showing that Na+ was not the principal charge carrier. It is concluded that plateau potentials, bistability and windup in turtle motoneurons do not depend on a CAN current even though its presence can be detected.